it depends on how many oldies you keep, I try to rehome ex-breeding does, but bucks end up hanging about forever. I've currently got 25 rats, but thats post two bouts of SDA. I usually have around 40 rats but I breed more than one variety. 15 rats for one self colour would be about right I would say.
I started with mice many many moons ago and thought that I would have to keep as many cages of rats as I did boxes of mice, but they are not the same, you also have much longer timescales for breeding which I am still getting used to in reverse with the mice again.
I use a soapy flannel to clean rats tails, I find green fairy liquid usually does the job
As a judge I am most grateful if rats nails are clipped. This is not an easy job for most and you will usually find someone wandering round at a show holding a rat and a pair of babies nail clippers with a pleading look on their face